How large is the Integer cache?

Question

Class Integer has cache, which caches the Integer values. So if I use method valueOf or inboxing the new value will not be instantiated, but get from the cache.

I know that the default cache size is 127 but can be extended due to VM settings. My question is: how large is the default value of cache size in these settings and can I manipulate this value? Is this value depended on which VM I use (32 or 64 bits)?

I'm now on tuning of a legacy code, and probably will need the conversion from int to Integer.

Clarification: Following code I've found in Java source

private static class IntegerCache {
    static final int low = -128;
    static final int high;
    static final Integer cache[];

    static {
        // high value may be configured by property
        int h = 127;
        String integerCacheHighPropValue =
            sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
        if (integerCacheHighPropValue != null) {
            int i = parseInt(integerCacheHighPropValue);
            i = Math.max(i, 127);
            // Maximum array size is Integer.MAX_VALUE
            h = Math.min(i, Integer.MAX_VALUE - (-low));
        }
        high = h;

        cache = new Integer[(high - low) + 1];
        int j = low;
        for(int k = 0; k < cache.length; k++)
            cache[k] = new Integer(j++);
    }

    private IntegerCache() {}
}

public static Integer valueOf(int i) {
    assert IntegerCache.high >= 127;
    if (i >= IntegerCache.low && i <= IntegerCache.high)
        return IntegerCache.cache[i + (-IntegerCache.low)];
    return new Integer(i);
}

So I think the cache is configurable.

Answer 1

Internal Java implementation and could not be configured, the range is from -128 to 127. You can check Javadocs or simply take a look at sources:

public static Integer valueOf(int i) {
         final int offset = 128;
         if (i >= -128 && i <= 127) { // must cache
             return IntegerCache.cache[i + offset];
         }
         return new Integer(i);
}

UPD. I was wrong (thx to Marco Topolnik). All of the above is related to older Java implementations. For Java 7 implementation could be achieved with system property:

-Djava.lang.Integer.IntegerCache.high=<size>

or JVM setting:

-XX:AutoBoxCacheMax=<size>

UPD. 2 java.math.BigInteger has hardcoded cache for values -16 <= x <= 16. From sources:

    private final static int MAX_CONSTANT = 16;
    private static BigInteger posConst[] = new BigInteger[MAX_CONSTANT+1];
    private static BigInteger negConst[] = new BigInteger[MAX_CONSTANT+1];
    static {
    for (int i = 1; i <= MAX_CONSTANT; i++) {
        int[] magnitude = new int[1];
        magnitude[0] = i;
        posConst[i] = new BigInteger(magnitude,  1);
        negConst[i] = new BigInteger(magnitude, -1);
    }
    }

    public static BigInteger valueOf(long val) {
        // If -MAX_CONSTANT < val < MAX_CONSTANT, return stashed constant
        if (val == 0)
            return ZERO;
        if (val > 0 && val <= MAX_CONSTANT)
            return posConst[(int) val];
        else if (val < 0 && val >= -MAX_CONSTANT)
            return negConst[(int) -val];
        return new BigInteger(val);
    }

Answer 2

For Java 7, I wasn’t able to find in Oracle’s java documentation is a description of exactly what would change when -XX:+AggressiveOpts is used. So I ran a process to determine the effect aggressiveopts has on jvm settings on one of our app servers in the lab that had java version: {quote} java version "1.7.0_15" Java(TM) SE Runtime Environment (build 1.7.0_15-b03) {quote}

The process is to run java with aggressiveopts disabled: java -d64 -server -XX:-AggressiveOpts -XX:+UnlockDiagnosticVMOptions -XX:+PrintFlagsFinal -version

The process is to run java with aggressiveopts enabled: java -d64 -server -XX:+AggressiveOpts -XX:+UnlockDiagnosticVMOptions -XX:+PrintFlagsFinal -version

This process produced these differences:

bool AggressiveOpts false --> true {product}

intx AutoBoxCacheMax 128 --> 20000 {C2 product}

intx BiasedLockingStartupDelay 4000 --> 500 {product}

bool EliminateAutoBox false --> true {C2 diagnostic}